Antenna for an implantable medical device

ABSTRACT

An apparatus and method for enabling far-field radio-frequency communications with an implantable medical device in which an antenna is embedded within a dielectric compartment of the device. A helical antenna may be employed to save space while still permitting far-field telemetry over a desired range of frequencies.

CROSS-REFERENCE TO RELATED APPLICATION(s)

This application is a continuation of U.S. patent application Ser. No.09/798,249, filed on Mar. 2, 2001, now issued as U.S. Pat. No.6,708,065, the specification of which is incorporated herein byreference.

FIELD OF THE INVENTION

This invention pertains to implantable medical devices such as cardiacpacemakers and implantable cardioverter/defibrillators. In particular,the invention relates to an apparatus and method for enablingradio-frequency telemetry in such devices.

BACKGROUND

Implantable medical devices, including cardiac rhythm management devicessuch as pacemakers and implantable cardioverter/defibrillators,typically have the capability to communicate data with a device calledan external programmer via a radio-frequency telemetry link. A clinicianmay use such an external programmer to program the operating parametersof an implanted medical device. For example, the pacing mode and otheroperating characteristics of a pacemaker are typically modified afterimplantation in this manner. Modern implantable devices also include thecapability for bidirectional communication so that information can betransmitted to the programmer from the implanted device. Among the datawhich may typically be telemetered from an implantable device arevarious operating parameters and physiological data, the latter eithercollected in real-time or stored from previous monitoring operations.

Telemetry systems for implantable medical devices utilizeradio-frequency energy to enable bidirectional communication between theimplantable device and an external programmer. An exemplary telemetrysystem for an external programmer and a cardiac pacemaker is describedin U.S. Pat. No. 4,562,841, issued to Brockway et al. and assigned toCardiac Pacemakers, Inc., the disclosure of which is incorporated hereinby reference. A radio-frequency carrier is modulated with digitalinformation, typically by amplitude shift keying where the presence orabsence of pulses in the signal constitute binary symbols or bits. Theexternal programmer transmits and receives the radio signal with anantenna incorporated into a wand which can be positioned in proximity tothe implanted device. The implantable device also generates and receivesthe radio signal by means of an antenna, typically formed by a wire coilwrapped around the periphery of the inside of the device casing.

In previous telemetry systems, the implantable device and the externalprogrammer communicate by generating and sensing a modulatedelectromagnetic field in the near-field region with the antennas of therespective devices inductively coupled together. The wand must thereforebe in close proximity to the implantable device, typically within a fewinches, in order for communications to take place. This requirement isan inconvenience for a clinician and limits the situations in whichtelemetry can take place.

SUMMARY OF THE INVENTION

The present invention is an apparatus and method for enablingcommunications with an implantable medical device utilizing far-fieldelectromagnetic radiation. Using far-field radiation allowscommunications over much greater distances than with inductively coupledantennas. Efficient emission and reception of far-field energy in adesirable frequency range, however, requires an antenna structure withcertain minimum dimensions. An objective of the present invention is toprovide such an antenna structure that does not complicate theimplantation procedure, does not interfere with the device at itsimplanted site, and is resistant to breakage or deformation due to bodymovements.

In accordance with the invention, a wire or other type of antenna isembedded in dielectric material and located within the unshielded headerof an implantable device where therapy leads are routed via feedthroughsto circuitry within the device housing. Alternatively, the embeddedantenna is located within an unshielded housing portion of animplantable device such as a dielectric pocket or window adjacent to therest of the housing. The antenna is connected to circuitry in order toenable the transmitting and receiving of far-field radio-frequencyradiation modulated with telemetry data. By containing the antenna inthis manner, the antenna is protected from bending or breakage andrequires no special implantation procedure.

The dimensions of an antenna structure contained within a device headeror housing portion are constrained by the size of those compartments,and it is desirable for implantable medical devices to be as small aspossible. In another aspect of the invention, a helically wound dipoleor monopole antenna is employed. A helical antenna is especiallyadvantageous for this purpose because it has a longer effectiveelectrical length than a similarly dimensioned straight wire dipole ormonopole antenna. The antenna may also be tuned with a tuning circuitthat optimizes its impedance for a particular frequency.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A through 1D illustrate different methods for compartmentalizingan embedded antenna within an implantable medical device.

FIGS. 2A and 2B illustrate alternative embodiments for connecting thecomponents of an exemplary cardiac rhythm management device to a helicalmonopole antenna.

FIGS. 3A and 3B illustrate alternative embodiments for connecting thecomponents of an exemplary cardiac rhythm management device to a helicaldipole antenna.

DETAILED DESCRIPTION

As noted above, conventional radio-frequency (RF) telemetry systems usedfor implantable medical devices such as cardiac pacemakers utilizeinductive coupling between the antennas of the implantable device and anexternal programmer in order to transmit and receive RF signals. Becausethe induction field produced by a transmitting antenna falls off rapidlywith distance, such systems require close proximity between theimplantable device and a wand antenna of the external programmer inorder to work properly, usually on the order of a few inches. Thepresent invention, on the other hand, is an apparatus and method forenabling telemetry with an implantable medical device utilizingfar-field radiation. Communication using far-field radiation can takeplace over much greater distances which makes it more convenient to usean external programmer. Also, the increased communication range makespossible other applications of the telemetry system such as remotemonitoring of patients and communication with other types of externaldevices.

A time-varying electrical current flowing in an antenna produces acorresponding electromagnetic field configuration that propagatesthrough space in the form of electromagnetic waves. The total fieldconfiguration produced by an antenna can be decomposed into a far-fieldcomponent, where the magnitudes of the electric and magnetic fields varyinversely with distance from the antenna, and a near-field componentwith field magnitudes varying inversely with higher powers of thedistance. The field configuration in the immediate vicinity of theantenna is primarily due to the near-field component, also known as theinduction field, while the field configuration at greater distances isdue solely to the far-field component, also known as the radiationfield. The near-field is a reactive field in which energy is stored andretrieved but results in no net energy outflow from the antenna unless aload is present in the field, coupled either inductively or capacitivelyto the antenna. The far-field, on the other hand, is a radiating fieldthat carries energy away from the antenna regardless of the presence ofa load in the field. This energy loss appears to a circuit driving theantenna as a resistive impedance which is known as the radiationresistance. If the frequency of the RF energy used to drive an antennais such that the wavelength of electromagnetic waves propagating thereinis much greater than the length of the antenna, a negligible far-fieldcomponent is produced. In order for a substantial portion of the energydelivered to the antenna to be emitted as far-field radiation, thewavelength of the driving signal should not be very much larger than thelength of the antenna.

An antenna most efficiently radiates energy if the length of the antennais an integral number of half-wavelengths of the driving signal. Adipole antenna, for example, is a center-driven conductor which has alength equal to half the wavelength of the driving signal. Such a dipoleantenna can be made of two lengths of metal arranged end to end with thecable from a transmitter/receiver feeding each length of the dipole inthe middle. An efficiently radiating resonant structure is formed ifeach length of metal in the dipole is a quarter-wavelength long, so thatthe combined length of the dipole from end to end is a half-wavelength.A shorter antenna can produce a similar field configuration by utilizinga ground plane to reflect electromagnetic waves emitted by the antennaand thereby produce an image field. A monopole antenna is a conductorwith a length equal to one-quarter the wavelength of the driving signalsituated with respect to a reflecting ground plane so that the totalemitted and reflected field configuration resembles that of the dipoleantenna. As will be discussed below, an antenna tuning circuit may alsobe used to alter the effective electrical length of an antenna byloading it with capacitance or inductance.

One way of implementing far-field telemetry in an implantable medicaldevice is to use an antenna that extends from the device housing. Thedevice housing is metallic and forms an electrically shieldedcompartment for electronic circuitry that provides particularfunctionality to the device such as cardiac rhythm management,physiological monitoring, drug delivery, or neuromuscular stimulation.The housing also contains circuitry for transmitting and receiving RFcommunications. The antenna could then take the form of a conductorcovered by insulation that extends from the housing and is electricallyconnected to the RF transmitter/receiver within the housing. The antennacould be any conductive structure capable of efficiently radiatingelectromagnetic energy well-known to those of skill in the art such as arod, a wire, a patch, or a loop.

An external wire antenna for an implantable medical device capable ofemitting far-field radiation, however, may require special implantationprocedures and may also be broken or deformed as a patient movesresulting in de-tuning. In accordance with the present invention,therefore, an antenna for an implantable medical device is embedded in adielectric and contained within a compartment of the implantable device.As described below with respect to specific embodiments, the compartmentmay take the form of a specialized dielectric pocket or window integralto the device housing or may be the unshielded device header.

As noted above, two common types of antennas are wire dipole andmonopole antennas. If a substantial portion of the RF energy deliveredto the antenna to be emitted as far-field radiation, the length of theantenna should not be very much shorter than one-quarter of thewavelength of the RF carrier signal provided by the RF transmitter. Forimplantable medical device applications, carrier frequencies between 300MHz and 1 GHz are most desirable. For example, the carrier signal may beselected to be 1 gigahertz, which corresponds to a wavelength ofapproximately 32 cm. A half-wavelength dipole antenna would optimally beapproximately 16 cm long, and a quarter-wavelength monopole antennawould optimally have a length approximately 8 cm with the housing 101serving as a ground plane. If it is desired to use a lower frequencycarrier, even longer antennas must be used. Depending upon the size ofthe implantable device, it may or may not be convenient to embed a wireantenna in a compartment of the device as described below. For reasonsof patient comfort, however, it is desirable for implanted devices to beas small as possible, and this constrains the length of the antenna thatcan be used if it is to be embedded in a compartment.

In another aspect of the invention, therefore, a helical antenna isemployed to transmit and receive RF signals. Such an antenna is formedby helically coiling a length of wire or other conductor along aparticular axis. If circumference of the individual helices small incomparison to the wavelength of the driving or received signal, theradiation pattern of the helical antenna is approximately the same aseither a dipole antenna or a monopole antenna if a ground plane ispresent. A helical dipole or monopole antenna may be formed by coiling alength of wire corresponding to just over one-half wavelength orone-quarter wavelength of the carrier frequency. Owing to the coiling ofthe wire, the resulting helical antenna is physically shorter than themonopole or dipole antenna formed from the straight piece of wire. Theeffective electrical length of a the helical antenna, however, is evenlonger than that owing to the added inductance of the coil andturn-to-turn capacitance which reduces the resonance frequency from thatof the corresponding straight wire antenna. A helical antenna thusprovides a shortened, space-saving monopole or dipole antenna thatbehaves electrically like a much longer antenna.

FIGS. 1A through 1D show different embodiments of an exemplaryimplantable cardiac rhythm management device with a compartmentalizedhelical antenna 200 suitable for radiating and receiving far-fieldelectromagnetic radiation. The device housing 102 is metallic andcontains therapy circuitry TC1 for providing particular functionality tothe device such as cardiac rhythm management, physiological monitoring,drug delivery, or neuromuscular stimulation as well as circuitry RFC1for providing RF communications. A battery B1 is used to supply power tothe electronic circuitry within the housing. One or more therapy leads310 are connected to the therapy circuitry contained within the housingby means of a header 103 with feedthroughs located therein for routingthe therapy leads to the appropriate internal components.

In FIGS. 1A and 1B, the helical antenna 103 is shown as being embeddedwithin the header 103. The header 103 is a solid block structure madefrom a synthetic polymer that has feedthroughs therein for routingelectrical connectors between the therapy leads 310 and the therapycircuitry TC1, with the proximal ends of the therapy leads beinginserted into bores 104. The polymer material of the header is adielectric that allows transmission and receipt of RF energy while alsoproviding a rigid covering that protects the antenna from bodily fluidsand prevents breakage or bending of the antenna. Putting the antennainto the header in this manner also involves no special implantationtechniques and leaves the physical profile of the device housingunaltered. FIG. 1A shows the antenna 200 positioned roughly parallel tothe surface of the device housing. In this configuration the helicallycoiled wire acts as a dipole antenna with a particular effectiveelectrical length. FIG. 1B shows an alternative embodiment in which theantenna 200 is positioned within the header perpendicular to the surfaceof the device housing. In this configuration, the conductive devicehousing 102 acts as a ground plane so that the helical coil acts as amonopole antenna and behaves with an electrical length twice that of asimilarly dimensioned dipole antenna.

FIG. 1C illustrates an alternative location for embedding the antennawithin the header. In this embodiment, an embedded conductor ishelically wound around one of the bores 104 into which an end of atherapy lead inserts. The antenna in this instance then acts as a dipoleantenna similar to that of FIG. 1A.

FIG. 1D shows another embodiment where the helical antenna, rather thanbeing located within the header, is embedded within a dielectric pocket105 adjacent an outer surface of the device housing. The pocket 105 maybe constructed of a polymer material similar to that of the headerdescribed above and provides similar protective and electricalfunctions. Again, the size and shape of the device housing is affectedminimally, if at all.

In the examples described above, the antenna embedded into either theheader or a dielectric pocket was a helical antenna. It should beappreciated, however, that other types of antennas could be similarlyembedded, and the same advantages would be obtained.

FIGS. 2A-B and 3A-B are block diagrams of an exemplary implantablecardiac rhythm management device showing examples of how monopole ordipole helical antennas may be connected and driven. In the figures,only one therapy lead 310 is shown but it should be understood that acardiac rhythm management device may use two or more such leads. Amicroprocessor controller 302 controls the operation of the therapycircuitry 320, which includes sensing and stimulus generation circuitrythat are connected to electrodes by the therapy leads for control ofheart rhythm, and RF drive circuitry 330 for transmitting and receivinga carrier signal at a specified frequency modulated with telemetry data.The conductors of the therapy lead 310 connect to the therapy circuitry320 through a filter 321 that serves to isolate the circuitry 320 fromany RF signals that may be picked up by the lead. The filter 321 may bea low-pass filter or a notch filter such as a choke.

The microprocessor 302 also outputs and receives the data contained inthe modulated carrier generated or received by the drive circuitry 330.The RF drive circuitry 330 includes an RF transmitter and receiver thatare connected by a transmit/receive switch 333 to the antenna. Theconductor that connects the transmit/receive switch to the antennapasses from the interior of the device housing to the exterior where theantenna is located through a feedthrough 404. One disadvantage ofhelical antennas is a relatively narrow bandwidth and low radiationresistance. An antenna tuning circuit may therefore be desirable, andall of the illustrated embodiments employ such a circuit to adjust theimpedance of the antenna. An antenna tuning circuit loads the antennawith a variable amount of inductance or capacitance to thereby adjustthe effective electrical length of the antenna, and hence the resonancefrequency of the antenna. By matching the antenna impedance to theimpedance of the transmitter/receiver at a specified carrier frequency,the reactance of the antenna may be tuned out at that frequency so thatthe antenna forms a resonant structure and efficientlytransmits/receives far-field radiation.

FIGS. 2A and 2B illustrate exemplary methods for connecting and drivinghelical monopole antennas such as the perpendicularly oriented helixshown in FIG. 1B. In FIG. 2A, the helical antenna 200 is simplyconnected to the transmit/receive switch 333 through a variable tuningcapacitor 402. In FIG. 2B, the antenna is connected to thetransmit/receive switch through a balun transformer 400 in addition tothe tuning capacitor. The balun transformer allows better impedancematching than when the tuning capacitor alone is used. The drivingimpedance may also be improved by tapping the antenna 200 in the middleas shown in the figure. The balun transformer also electrically isolatesthe internal circuitry from the device housing which may be advantageousin some pacemakers and defibrillators where the housing or can isutilized as an electrode in delivering pacing or defibrillation pulses.

FIGS. 3A and 3B show exemplary methods for connecting and drivinghelical dipole antennas such as the parallel oriented helices shown inFIGS. 1A, 1C, and 1D. In these embodiments the balun transformer servesto convert the single ended RF signal generated or received by thecircuitry into a differential RF signal that is transmitted or receivedat the antenna in addition to providing improved impedance matching.FIG. 3A shows an embodiment where the balun transformer 400 isinterposed between the tuning capacitor and the antenna 200 and alsoprovides electrical isolation. FIG. 3B is an alternative embodimentwhere the antenna itself serve as one of the windings of the baluntransformer 400. This saves space but suffers from a disadvantage thatthe balun is outside of the housing and hence cannot be used to isolatethe internal circuitry from the housing.

Although the invention has been described in conjunction with theforegoing specific embodiment, many alternatives, variations, andmodifications will be apparent to those of ordinary skill in the art.Such alternatives, variations, and modifications are intended to fallwithin the scope of the following appended claims.

1. An implantable medical device, comprising: a housing for containingelectronic circuitry; an antenna embedded in a dielectric compartment;circuitry within the housing connected to the antenna for transmittingand receiving a modulated radio-frequency carrier at a specified carrierfrequency; and, an antenna tuning circuit for matching the impedance ofthe antenna to the transmitting/receiving circuitry at a specifiedcarrier frequency by loading the antenna with inductance or capacitanceand a transformer through which the antenna is connected to thetransmitting/receiving circuitry for electrically isolating thetransmitting/receiving circuitry from the housing, wherein the antennatuning circuit further comprises a variable tuning capacitor foradjusting the resonant frequency of the antenna.
 2. The device of claim1 wherein the dielectric compartment is within a header for the devicehaving feedthroughs therein for routing connections between internalcircuitry and external leads.
 3. The device of claim 1 wherein thedielectric compartment is a dielectric pocket adjacent a surface of thedevice housing.
 4. The device of claim 1 wherein the dimensions of theantenna are such that a significant portion of radio-frequency energydelivered to the antenna at the specified carrier frequency is emittedas far-field radiation.
 5. The device of claim 1 wherein the antenna isa helically coiled antenna.
 6. The device of claim 5 wherein thehelically coiled antenna is oriented roughly parallel to a surface ofthe device housing and further wherein the electrical length of theantenna is approximately one-half wavelength of the specifiedradio-frequency carrier.
 7. The device of claim 5 wherein the helicallycoiled antenna is oriented roughly perpendicular to a surface of thedevice housing and further wherein the electrical length of the antennais approximately one-quarter wavelength of the specified radio-frequencycarrier so as to act as a monopole antenna with the device housingserving as a ground plane.
 8. The device of claim 1 wherein thetransformer is a balun transformer for converting between a single-endedsignal generated or received by the transmitter/receiver circuitry and adifferential signal generated or received by the antenna.
 9. The deviceof claim 8 wherein a winding of the balun transformer is formed by thehelical antenna.
 10. The device of claim 1 wherein the device is acardiac rhythm management device having rhythm control circuitryelectrically connected to one or more electrodes adapted for dispositionwithin or near the heart by one or more therapy leads.
 11. The device ofclaim 10 wherein the helical antenna is embedded within a header of thedevice coiled around a bore into which an end of a therapy lead isinserted.
 12. A method for transmitting and receiving radio-frequencysignals in an implantable medical device, comprising: transmitting orreceiving a modulated radio-frequency carrier at a specified carrierfrequency to or from a helically coiled antenna; matching the impedanceof the antenna to the transmitting/receiving circuitry at a specifiedcarrier frequency by loading the antenna with inductance or capacitanceusing an antenna tuning circuit; and, connecting the antenna to thetransmitting/receiving circuitry though a transformer to electricallyisolate the transmitting/receiving circuitry from a device housing. 13.The method of claim 12 further comprising converting between asingle-ended signal generated or received by the transmitter/receivercircuitry and a differential signal generated or received by the antennawith a balun transformer.
 14. The method of claim 13 further comprisingadjusting the resonant frequency of the antenna to a specified carrierfrequency with a variable capacitor.
 15. The method of claim 12 whereinthe helically coiled antenna is oriented roughly parallel to a surfaceof the device housing and further comprising transmitting at a carrierfrequency with a wavelength approximately twice the electrical length ofthe antenna.
 16. The method of claim 12 wherein the helically coiledantenna is oriented roughly perpendicular to a surface of the devicehousing and further comprising transmitting at a carrier frequency witha wavelength approximately four times the electrical length of theantenna such that the antenna acts as a monopole antenna with the devicehousing serving as a ground plane.
 17. The method of claim 12 whereinthe transformer is a balun transformer and further comprising convertingbetween a single-ended signal generated or received by thetransmitter/receiver circuitry and a differential signal generated orreceived by the antenna with the balun transformer and wherein onewinding of the transformer is formed by the helical antenna.
 18. Animplantable medical device, comprising: a housing for containingelectronic circuitry; control circuitry electrically connected to one ormore electrodes adapted for disposition in a patient's body by one ormore therapy leads a helical antenna embedded in a dielectriccompartment; circuitry within the housing connected to the antenna fortransmitting and receiving a modulated radio-frequency carrier at aspecified carrier frequency; and, wherein the helical antenna isembedded within a header of the device coiled around a bore into whichan end of a therapy lead is inserted.
 19. The device of claim 18 furthercomprising an antenna tuning circuit for matching the impedance of theantenna to the transmitting/receiving circuitry at a specified carrierfrequency by loading the antenna with inductance or capacitance and abalun transformer through which the antenna is connected to thetransmitting/receiving circuitry wherein the antenna tuning circuitfurther comprises a variable tuning capacitor for adjusting the resonantfrequency of the antenna.
 20. The device of claim 18 wherein the deviceis a cardiac rhythm management device having rhythm control circuitryelectrically connected to one or more electrodes adapted for dispositionwithin or near the heart by the one or more therapy leads.